As generally known, a typical aftertreatment device for exhaust gas discharged from a diesel engine includes both a soot filter that collects particular matter in the exhaust gas and a urea SCR device that purifies a nitrogen oxide (NOx) in the exhaust gas with a urea solution (see, for instance, Patent Literature 1).
A diesel engine used in a construction machine has a large displacement as compared with that of a diesel engine used in a transport truck or the like, and thus requires a large-sized aftertreatment device capable of dealing with such a large displacement.
However, since an aftertreatment device is usually provided in an engine compartment of a construction machine, such a large-sized aftertreatment device cannot simply be installed due to a limited installation space. Accordingly, it is disclosed that a plurality of aftertreatment devices with a typical size are installed to ensure a sufficient performance, the aftertreatment devices being arranged in such a manner that a narrow space is efficiently used (see, for instance, Patent Literature 2).